L-selectin is a cell surface adhesion molecule whose functions include mediation of leukocyte-endothelial interactions during inflammation and the "homing" of lyphocytes to peripheral lymph nodes. The involvement of L-selectin in lymphocyte trafficking suggests its role in the migration of stem cells during fetal hematopoiesis or with certain leukemias. While the role of L-selectin in mature neutrophil function is well-established, its role in hematopoiesis has not been extensively studied. It is also likely that L-selectin is developmentally-regulated, as impairment of L- selectin-mediated function has been observed in cord blood neutrophils. We hypothesize that: 1) L- selectin is regulated during fetal and neonatal development and 2) L-selectin plays an influential role during normal and dysregulated hematopoiesis. The long term objectives of this proposal are: 1) To define the developmental regulation of functional L-selectin; and, 2) To investigate the role played by L-selectin during normal and dysregulated hematopoiesis. The specific aims outlined in this proposal are structured in the following way: first, to investigate L-selectin in the mature neutrophil; secondly, to evaluate L-selectin in the early cells which give rise to mature neutrophils, the progenitor cells; and thirdly, to assess the role played by L-selectin in the interaction of those progenitors with the stromal environment during hematopoiesis. Mature neutrophils and neutrophil progenitors from human subjects, which include fetuses and newborns, healthy adults, and patients with myeloid leukemia, will be investigated using flow cytometric analysis and functional adhesion assays. In this way the influence of normal and dysregulated development of L-selectin function and expression can be investigated. Animal studies involving postmortem murine tissue as well as tissue immortalized in cell lines at various developmental stages will be analyzed to determine both the composition of the stroma and the influence of developmental age on antigenic expression. Analytical methods in these studies will include measurement of protein expression using flow cytometry and enzyme-linked immunosorbent assays, molecular biology techniques to evaluate mRNA expression, and functional adhesion assays.